The present invention relates to thermal recording materials that produce color in response to heat.
Thermal recording materials, as a class, are well known. The recording material generally comprises a support carrying a color-forming composition that is thermally sensitive; i.e., changes color upon sufficient heating. The color-forming composition has two main components: a color-forming dye (electron-donating dye precursor), also known as a leuco dye, and an acidic developer. The leuco dye and acidic developer are usually dispersed in a binder. Sufficient heating will permit the acidic developer to react with the leuco dye which results in the formation of a color at the site of the heating. This basic system is described in many patents including U.S. Pat. Nos. 3,539,375; 3,674,535; 3,746,675; 4,151,748; 4,181,771; 4,246,318; 4,470,057; and 5,955,398.
In typical thermal systems, in addition to the leuco dye and developer, the color-forming composition may also contain another material that aids in color formation; sometimes called a modifier. These additional material(s) can function by lowering the melting point of the dye/developer and/or by acting as a type of solvent in which the dye and developer dissolve. In this way, the reaction between a leuco dye and a developer is often more easily facilitated. The result is the formation of a more intense image and/or faster imaging. See for example, U.S. Pat. Nos. 4,531,140; 4,794,102; 5,098,882; 6,835,691; and 6,921,740.
A variety of acidic developers are described in the art including phenolic compounds; e.g., monophenols and diphenols, as taught in U.S. Pat. Nos. 3,539,375 and 6,566,301.
It is desirable to find new modifiers and developer systems, especially ones that could be more environmentally friendly.